Restart the computer and while booting up press and hold the F8 key on the keyboard. If you can't even get into safemode run a check/scan disk. Grab a 98 boot disk from
here and run a check disk. If you can boot into it it is proberly a bad/corrupt driver or module loaded. Try to uninstall some drivers, programs.
More over check the processor and make sure it did not hit the thermal peak. If it does it can't function under load stress and will cause the computer to frezzzzz real bad ;(. Other than that check the RAM and lastly power supply. I am sure that it is the OS(software issue though their is all ways a chance it is hardware)
Take a look
HERE
Its a MS KB article